Field of the Invention
The present invention relates to an apparatus for inspection of packaged printed circuit boards and more particularly to a packaged printed circuit board inspection apparatus in which a fine light beam, finely focused, is irradiated on a packaged printed circuit board and reflection beams therefrom are detected by using the principle of trigonometrical survey to inspect the positional shift, loss and defective soldering of packaged parts.
Description of the Related Art
In recent years, for inspection of the positional shift, loss and defective soldering of parts of a packaged printed circuit board, a non-contact type inspection apparatus based on the principle of trigonometrical survey has been used in which a finely focused light beam is irradiated on the packaged printed circuit board and reflection beams therefrom are detected.
Prior art will be described hereunder with reference to the accompanying drawings. FIG. 25 shows an example of a conventional packaged printed circuit board inspection apparatus. In the drawings, light beams will be all represented by optical axes. In FIG. 25, a light source 120 generates a fine light beam to be irradiated on a packaged printed circuit board 126, a collimator lens system 121 converts the fine light beam into a parallel flux of light, a polygon mirror 122 deflects the parallel light flux, a projection lens system 125 irradiates the parallel light flux deflected by the polygon mirror 122 onto the packaged printed circuit board 126 vertically thereto, a mirror 124 guides the parallel light flux deflected by the polygon mirror to the projection lens system 125, light receiving lens systems 127, 128, 129 and 130 collect reflection beams from the packaged printed circuit board 126, and photoelectric conversion devices 131, 132, 133 and 134 measure light receiving positions at which the reflection beams collected by the light receiving lens systems 127, 128, 129 and 130 are received by the devices 131 to 134. A polygon motor 123 rotates the polygon mirror 122 to change the beam irradiation position of the fine light beam on the packaged printed circuit board 126, a table 135 moves the packaged printed circuit board 126 in a direction of arrow y in accordance with the optical scanning (in a direction of arrow x), a ball screw 136 is driven for rotation to move the table 135, a motor 137 rotates the ball screw 136, and guide rails 138 are adapted to guide the table 135.
The operation of the packaged printed circuit board inspection apparatus constructed as above will be described. A fine light beam emitted from the light source 120 is converted by the collimator lens system 121 into a parallel light flux which is deflected by the polygon mirror 122 and mirror 124 to pass through the projection lens system 125 and irradiate the packaged printed circuit board 126 vertically thereto. The polygon mirror 122 is driven for rotation by the polygon motor 123 and as the polygon mirror 122 rotates, the fine light beam is scanned on the packaged printed circuit board 126 in the direction of arrow x.
Reflection beams of the fine light beam which return from the packaged printed circuit board 126 are collected by the light receiving systems 127, 128, 129 and 130 arranged in four directions and are irradiated on the photoelectric conversion devices 131, 132, 133 and 134 at positions corresponding to a height of a scanning position of the fine light beam on the packaged printed circuit board 126. The irradiation positions are detected on the basis of electrical outputs of the photoelectric conversion devices 131, 132, 133 and 134 and the shape of the surface of the packaged printed circuit board is measured through the principle of trigonometrical survey. By moving the table 135 in the direction of arrow y vertical to the scanning direction by means of the motor 137, ball screw 136 and guide rails 138, the three-dimensional shape of the entire surface of the packaged printed circuit board 126 can be measured.
However, in the prior art construction as above, as the scanning position changes, the light receiving angle for the reflection beam and the collecting position for the reflection beam on the photoelectric conversion device change even when the height of an object to be measured remains unchanged. This phenomenon will be described with reference to FIG. 26. FIG. 26 is a side view of FIG. 25 as seen in the direction of arrow y, showing light receiving positions a and b for reflection beams on the photoelectric conversion devices when the scanning position changes from A to B. For simplicity of explanation, the mirror 124 is omitted. As is clear from FIG. 26, even when the same surface height is measured, the light receiving position and collecting position for the reflection beams on each of the photoelectric conversion devices change greatly as the scanning position changes.
Therefore, each of the photoelectric conversion devices 131, 132, 133 and 134 needs, in addition to a light receiving range of reflection beams necessary for height measurement, a light receiving range for covering a change in the light receiving position for reflection beams due to a change in the scanning position, and for simultaneous measurement over a wide range, photoelectric conversion devices each having a wide light receiving area are required. In general, the photoelectric conversion device has such characteristics that as the light receiving area expands, the response speed is degraded and therefore it is difficult to measure the three-dimensional shape of a measuring object of wide range at a high speed.
Further, because of the phenomenon that the light receiving position changes with the scanning position, the height must be corrected in accordance with scanning positions. Accordingly, the resolution of the sensor per se constructed of the photoelectric conversion devices and processing circuits cannot be used exclusively for height measurement and the accuracy is deteriorated.
In addition, the light receiving angle changes with the scanning position, raising a problem that characteristics of the triogonometrical survey, for example, the state of the dead angle (a portion which has an irregularity in its surface to intercept reflection beams and cannot be measured through trigonometrical survey) changes with the scanning position.